Two previous approaches are known for adapting a hearing aid device to an individual feedback situation. With a first approach, the actual feedback path or its characteristics is not known and adaptation is an ongoing process, with the danger of artifacts being very high. According to the second approach, the actual feedback path is first measured by an adapting computer or a programming device. Basic values for the feedback reduction are produced from this measurement and only a minimal ongoing adaptation still needs to be carried out by the hearing device based on these basic values or this fine adaptation is dispensed with altogether. Since a special adapting computer is necessary for measuring the feedback path, the measurement is only carried out by an acoustician and never however by a hearing aid wearer themselves.
U.S. Pat. No. 4,783,818 discloses a method and a device for the adaptive filtering of acoustic feedback signals. In order to identify the acoustic feedback parameters, the configuration of the communication system is changed from a conventional operating mode into a parameter identification mode. In this case, the amplifier is conventionally separated from the microphone and the loudspeaker and is replaced by an identification circuit. Automatic toggling between the two modes takes place as a function of a threshold value. The disadvantage here is that under some circumstances the hearing device fails very frequently during normal hearing operation, namely whenever it switches to identification mode.
Publications EP 0 415 677 B1, EP 0 634 084 B1 and WO 94/09604 also disclose hearing aids with feedback suppression. In this case own noise sources are partially used.
Furthermore, a digital hearing aid is described in the patent application DE 41 28 172 C2, in which an acoustic sensor detects an otoacoustic response of the inner ear of the hearing aid wearer to the measuring tones emitted by an electroacoustic converter. With the aid of the relevant sensor voltage, a microcomputer carries out a comparison and a correction of the transmission function. The measuring and correction process is initiated for instance by activating a key connected to the microcomputer, said key having the function of a switch.
The article Bisgaard, Nikolai; Dyrlund, Ole; “DFS—ein neues digitales System zur Rückkopplungsunterdrückung in Hörgeräten”; (DFS a new digital system for feedback suppression in hearing devices) in Audiological Acoustics 5/91, pages 166, 168 and 173-177 discloses a digital system for feedback suppression in hearing devices. The feedback signal is defined here as a part of the output signal which is fed to the microphone. The feedback either takes place via a ventilation flute or a leakage between the ear canal piece and the auditory canal. If the attenuation on the path of the feedback signal is lower than the amplification of the hearing device, oscillation with a very high acoustic pressure occur in the system. The digital feedback suppression system constantly measures the frequency curve of the feedback path and generates a correction system which is superimposed with the input signal in order to suppress the feedback. This system nevertheless also includes the above-mentioned disadvantages of a constantly adapting system.